The present invention relates to a charge control circuit, and more particularly, the present invention relates to a charge control circuit for controlling the charging of at least one battery by a charging source, such as a photovoltaic array.
In general, charge control circuits are used to selectively connect and disconnect a charging source to a battery. An example of one such charge control circuit is disclosed in U.S. Pat. No. 4,626,764, entitled "Photovoltaic Battery Charge Controller" and issued to Weinhardt on Dec. 2, 1986 (hereinafter referred to as "the '764 patent"). The charge control circuit disclosed in this patent is shown in FIG. 1 and includes a relay switch 14 having a fixed terminal coupled to a positive terminal 12 of a photovoltaic panel 11, a normally-closed contact terminal 15 coupled to an anode of a reverse current blocking diode 21, a normally-open contact terminal 16 coupled to an indicator lamp 19, and a relay coil 17 coupled to the positive terminal 12 of the photovoltaic panel 11 at one end and coupled to a first terminal of a variable resistor 18 at the other end. The variable resistor 18 has a second terminal coupled to both the negative terminal 13 of the photovoltaic panel 11 and the negative terminal 23 of the battery 24. The positive terminal 22 of the battery 24 is coupled to the cathode of the reverse current blocking diode 21.
In operation, when solar energy impinges upon the photovoltaic panel 11, the charge control circuit disclosed in the '764 patent allows the current generated by the photovoltaic panel 11 to pass from positive terminal 12 through switch 20 via the normally-closed contact terminal 15 of relay switch 14, through diode 21, and into the positive terminal 22 of battery 24. The remaining loop of the charging circuit is provided by coupling the negative terminal 23 of battery 24 to the negative terminal 13 of photovoltaic panel 11. Since coil 17 is in series with variable resistor 18, the amount of current flowing through coil 17 is regulated by the resistance imposed by variable resistor 18. In addition, the voltage drop/potential difference across coil 17 and variable resistor 18 is effectively the same as the potential difference across battery terminals 22 and 23. Therefore, if battery 24 is to be charged to a certain potential difference, variable resistor 18 can be adjusted such that a sufficient amount of current is allowed to flow through coil 17 creating the requisite amount of magnetic flux to cause switch 20 of relay switch 14 to cross from the normally-closed contact 15 to normally-open contact terminal 16, thereby opening the battery charging circuit, the third closed circuit, and preventing further charge to battery 24.
When switch 20 connects the fixed contact with the normally-open contact, a current loop passes from the positive terminal 12 of photovoltaic panel 11 through relay coil 17, variable resistor 18, and back to the negative terminal 13 of photovoltaic panel 11. The constant current through relay coil 17 causes relay switch 14 to remain in its normally-open position until no further solar energy is received by photovoltaic panel 11. Thus, when the sun sets, relay switch 14 returns to its normally-closed position making a connection between battery 24 and photovoltaic panel 11. To prevent battery 24 from subsequently discharging through photovoltaic panel 11, reverse current blocking diode 21 must be provided between the positive terminals of battery 24 and photovoltaic cell 11. Utilizing a reverse current blocking diode having a sufficient rating for blocking large amounts of current flowing from the battery is disadvantageous because such diodes consume a significant portion of the charging energy supplied to the battery during a charging cycle resulting in a loss in efficiency.
Further, the charge control circuit disclosed in the '764 patent and depicted in FIG. 1 is specifically designed for its simplicity of construction and low cost. However, this charge control circuit is extremely inefficient for several reasons. First, the charge control circuit only allows one charging cycle per day. Thus, if battery 24 is discharged during the day, the charge control circuit will not permit battery 24 to be recharged that day even though there may be an abundance of daylight left to fully recharge the battery. Second, by disconnecting a battery from a charging source when the battery voltage reaches a preset voltage level, such as the plate saturation point (i.e., the battery voltage at which the battery is bubbling), the charge control circuit disclosed in the '764 patent will only charge the battery to 20 to 50% of its capacity unless the charging current is very small compared to the battery's amp/hour capacity. Once a battery voltage levels off at the plate saturation point, the battery may be fully charged by subsequently intermittently charging the battery by connecting and disconnecting the charging source. The charge control circuit disclosed in the '764 patent would not allow for such intermittent charging to full battery capacity.
Other, more sophisticated and complex charge control circuits utilize a comparator circuit, which senses the voltage of the battery, compares the sensed voltage to a first reference voltage level, and energizes a relay coil of a relay switch coupled between the positive terminals of a photovoltaic panel and a battery when the comparator determines that the sensed battery voltage is below the first reference voltage in order to charge the battery. In this manner, the battery will be recharged whenever its voltage drops below the predefined first reference voltage level thus permitting the battery to be charged more than once per day if necessary. These comparator circuits also compare the sensed battery voltage to a second, higher reference voltage level and de-energize the relay coil to disconnect the battery from the charging source when the comparator determines that the sensed battery voltage exceeds the predefined second reference voltage level. In this manner the charge control circuit may prevent overcharging of the battery. As stated above, the comparator circuits energize the relay coil of the relay switch when in a charging state. Thus, the battery and the charging source are disconnected when the relay switch is in the normally-closed position. By energizing the relay coil during charging, the charge control circuits must divert some of the charging current through the relay coil resulting in inefficient use of the charging current during a charging state.
Although a charge control circuit utilizing the normally-closed position of a relay switch to connect a battery to a charging source is known, as disclosed in the '764 patent, the specific implementation of such a relay switch as disclosed was not recognized as providing increased efficiency particularly in view of the other noted reasons why the charge control circuit of the '764 patent is inefficient. Further, the specific implementation of the relay switch in the '764 patent was employed to allow for use of the simplified switching control mechanism, but not to increase the efficiency of the charge control circuit.
Conventional charge control circuits are often implemented with a low battery load disconnect circuit for disconnecting a load from the battery when the battery voltage falls below a preset voltage level. Such low battery load disconnect circuits do not reset (i.e., reconnect the load to the battery) until the battery has been fully charged or charged to a predetermined voltage, which is generally set just above the no load resting voltage of a charged battery (about 12.7 V for a 12 V battery). Thus, the load will remain disconnected in the morning even though the photovoltaic panels are illuminated and the battery is nearly charged. Further, on days in which the battery did not charge enough to reset the low battery load disconnect reset voltage, the load will remain disconnected at night even though the battery may be charged sufficiently to provide power to the load.
Additionally, most conventional charge control circuits utilize two predefined set points for connecting and disconnecting the battery from the charging source. However, depending upon the desired battery charging voltage, these set points should be changed to optimize the charging for the particular charging voltage.